1. Field of the Invention
The present invention relates to a method of preparing toner and the toner, and a developer and an image forming method using the toner.
2. Discussion of the Background
Developers for use in electrophotography, electrostatic recording and electrostatic printing, in their development processes, are transferred to image bearers such as photoreceptors electrostatic latent images are formed on, transferred therefrom to a transfer medium such as a transfer paper, and fixed thereon. As the developers for developing electrostatic latent images formed on image bearers, a two-component developer including a carrier and a toner, and a one-component developer without a carrier, such as a magnetic toner and a nonmagnetic toner, are known.
Conventionally, as a dry toner for use in electrophotography, electrostatic recording and electrostatic printing, a pulverized toner is widely used, which is formed by kneading a toner binder such as styrene resins and polyester resins with a colorant, etc. upon application of heat, cooling the kneaded mixture to be solidified and pulverizing the solidified mixture.
Recently, polymerized toners prepared by suspension polymerization methods, emulsion polymerization condensation methods, etc. are being used. Besides, Japanese published unexamined application No. 7-152202 discloses a polymer solution suspension method of using a volume contraction. This method includes dispersing or dissolving toner constituent in a volatile solvent such as an organic solvent having a low boiling point to prepare a dispersion or a solution, emulsifying the dispersion or solution in an aqueous medium to form a droplet, and removing the volatile solvent. The diversity of resins this method can use is wider than those of the suspension polymerization methods and emulsion polymerization condensation methods, and has an advantage of being capable of using a polyester resin effectively used for full-color images requiring transparency and smoothness after fixed.
However, in the polymerization methods, since a dispersant is basically used in an aqueous medium, the dispersant impairing the chargeability of a toner remains on the surface thereof, resulting in deterioration of environmental resistance. In addition, a large amount of water is needed to remove the dispersant, resulting in unsatisfactory methods of preparing a toner.
Japanese published unexamined application No. 2003-262976 discloses a method and an apparatus forming a microscopic droplet of a toner constituent with a piezoelectric pulse, and drying and solidifying the microscopic droplet to form a toner. Further, Japanese published unexamined application No. 2003-280236 discloses a method of forming a microscopic droplet thereof with a heat expansion in a nozzle, and drying and solidifying the microscopic droplet to form a toner. Furthermore, Japanese published unexamined application No. 2003-262977 discloses a method of forming a microscopic droplet with an acoustic lens, and drying and solidifying the microscopic droplet to form a toner. However, these methods discharge a droplet only from one nozzle, and the number of the droplets dischargeable per unit of time is small, resulting in poor productivity. At the same time, the droplets are inevitably combined each other, resulting in a wide particle diameter distribution and low mono-dispersibility.
Japanese published unexamined applications Nos. 2006-28432 and 2006-28433 disclose a method of intermittently discharging a dispersion in which toner materials including a thermosetting resin or a UV curable resin are finely dispersed in a dispersion medium to form a droplet, agglutinating the droplet and hardening the thermosetting resin or the UV curable resin to stabilize formation of a particle. However, these methods have low productivity and insufficient mono-dispersibility as the above-mentioned Japanese published unexamined applications Nos. 7-152202, 2003-262976, 2003-280236 and 2003-262977. Although the resins are hardened after a particle is formed, the properties after fixed are not satisfactory.
The methods disclosed in Japanese published unexamined applications Nos. 2006-28432 and 2006-28433 directly contact the oscillator to a fluid. In such a constitution, the resultant toner has a sharp particle diameter distribution when the numbers of the orifices and oscillators are same. However, when the constitution has one oscillator and many orifices, the sizes of droplets vary depending on distances between the orifices and the oscillator, resulting in production of toners having different particle diameters among plural orifices.
These dry toners are typically contacted, heated, melted and fixed on a paper, etc. with a heated roll or belt after developed and transferred onto the paper because of its high heat efficiency. When the heated roll or belt has too high a temperature, the toner is melted so excessively that the toner is bonded with the heated roll or belt, i.e., a hot offset problem. When the heated roll or belt has too low a temperature, the toner is not fully melted and fixed on the paper. In terms of saving energy and downsizing the apparatus, a toner having a higher hot offset generation temperature (good hot offset resistance) and a low fixable temperature (good low-temperature fixability) is desired. In addition, a toner needs thermostable preservability as well to avoid blocking at environmental temperatures in a container and an apparatus. Above all, since full-color images need glossiness and mixability, the toner needs a lower melting viscosity and a toner binder having sharp meltablity is used therein.
However, such a toner is likely to have a hot offset problem, and silicone oils are conventionally applied to heat rolls in full-color image forming apparatuses. However, the full-color image forming apparatuses need oil tanks and oil applicators to apply oils to the heat rolls and become complicated and large. The oils inevitably adhere to copy papers and OHP films, resulting in deterioration of writability of an aqueous ink and color toner on the OHP film.
In order to prevent the hot offset without applying an oil to the heat roll, a release agent such as a wax is added to the toner, or a polymeric or crosslinking material is introduced to a binder resin to increase the viscoelasticity when melted.
However, a toner constituent liquid including a binder resin the polymeric or crosslinking material is introduced to is very difficult to discharge from a nozzle having microscopic orifices with a mechanical oscillator.
Particularly, recent toners tend to have small particle diameters to produce high-definition and high-quality images, and it is more difficult to produce toners having both offset resistance and dischargeability from a nozzle.
Because of these reasons, a need exists for a method of efficiently preparing a toner having good offset resistance, better mono-dispersibility than ever before and no or scarce variation of image quality with a toner constituent liquid having good dischargeability from a nozzle.